1. Field of the Invention
The present invention refers to the field of power plant technology. It relates to a method for operating a (stationary) gas turbine, as well as to a gas turbine for implementing the method.
2. Brief Description of the Related Art
A gas turbine with reheating (reheat gas turbine) is known (see, for example, U.S. Pat. No. 5,577,378 or “State of the art gas turbines—a brief update,” ABB Review February, 1997, FIG. 15, turbine type GT26), which combines flexible operation with very low flue gas emission readings.
The principle of the known gas turbine with reheating is shown in FIG. 1. The gas turbine 11, which is a portion of a combined cycle power plant 10, includes two connected compressors, arranged behind one another on a common shaft 15, namely a low pressure compressor 13 and a high pressure compressor 14, as well as two combustors, namely a high pressure combustor 18 and a reheat combustor 19, and the pertinent turbines, namely a high pressure turbine 16 and a low pressure turbine 17. The shaft 15 drives a generator 12.
The manner in which the unit works is as follows: air is drawn in via an air inlet 20 from the low pressure compressor 13, and is compressed initially to a level of intermediate pressure (ca. 20 bar). The high pressure compressor 14 then further compresses the air to a level of high pressure (ca. 32 bar). Cooling air is diverted at both the level of intermediate pressure and at the level of high pressure and cooled down in pertinent OTC coolers (OTC=Once Through Cooler) 23 and 24 and conducted further to the combustors 18 and 19 and turbines 16, 17 via cooling lines 25 and 26 for cooling purposes. The remaining air from the high pressure compressor 14 is led to the high pressure combustor 18 and heated there by the combustion of a fuel, which is introduced via the fuel feedline 21. The resultant flue gas is then expanded in the subsequent high pressure turbine 16 to an intermediate level of pressure, as it performs work. After expansion, the flue gas is heated again in the reheat combustor 19 by means of the combustion of a fuel introduced via fuel feedline 22 before it is expanded in the subsequent low pressure turbine 17, performing additional work in the process.
The cooling air, which flows through the cooling lines 25, 26, is blown in at suitable points of combustors 18, 19 and turbines 16,17, in order to limit the material temperatures to a reasonable extent. The flue gas that comes from the low pressure turbine 17 is sent through a heat recovery steam generator (HRSG) 27, in order to produce steam, which flows through a steam turbine 29 within a water-steam circuit, performing additional work there. After flowing through the heat recovery steam generator 27, the flue gas is finally released to the outside through a flue gas line 28. The OTC coolers 23, 24 are part of the water-steam circuit; super-heated steam is produced at their outlets.
As a result of the two combustions in combustors 18, 19, which are dependent upon one another and follow one another sequentially, a great flexibility in operation is achieved; the combustion temperatures can be adjusted so that the maximum degree of effectiveness is achieved within the existing limits. The sequential combustion system's low flue gas values are the result of the inherently low emission values that can be achieved in conjunction with reheating.
On the other hand, combined cycle power plants with single-stage combustion in the gas turbines are known (see, for example, U.S. patent application Ser. Nos. US-A -4,785,622 or US-B2-6,513,317), in which a coal gasifier is integrated, in order to provide the requisite fuel for the gas turbine in the form of syngas, which is recovered from coal. Such combined cycle power plants are referred to as IGCC plants (IGCC=Integrated Gasification Combined Cycle).
The present invention now proceeds from the recognition that by using gas turbines with reheating in an IGCC unit, the advantages of this type of gas turbine can be rendered usable for the unit in a particular manner.